Epidemic and pandemic influenza occurs annually and is a cause of significant morbidity and mortality worldwide. Influenza viruses are highly pleomorphic particles composed of two surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA). The HA mediates attachment of the virus to the host cell and viral-cell membrane fusion during penetration of the virus into the cell. Therefore, an influenza vaccine typically contains an effective amount of HA that matches the diseases strain to induce production of neutralizing antibodies against the disease strain.
Single-radial-immunodiffusion (SRID) assays have been used to determine the Hemagglutinin (HA) content in the influenza vaccine and to measure potency of influenza virus vaccines licensed by the Food and Drug Administration for use in the United States since 1978. Specifically, SRID determines HA content in an influenza vaccine by using specific anti-HA antibodies. Samples of vaccines are applied onto an agar plate containing a strain-specific antiserum. The plates are typically incubated in a moist chamber at room temperature to allow diffusion of the antigen. Reaction of the antigen with the antibody produces a zone of precipitation (which is in form of precipitation ring). The amount of HA in the vaccine samples can be quantified by comparing the ring diameters of samples with the diameters of known concentrations of the reference HA protein. A potency value for the vaccine tested can be obtained based on the amount of HA.